Conventionally, polylactic acid, which is one of bioplastics, has been widely used in various fields such as the fields of medical materials and agricultural materials. However, the use of polylactic acid itself is limited due to its low impact resistance and flexibility, and it is desired to improve the use as a resin composition in combination with an additive. For example, in order to be used for automobile parts, members of household electric products, and the like, polylactic acid is required to have thermoplasticity, as well as improved impact resistance and flexibility.
Meanwhile, the copolymerization and the alloying of polylactic acid, and the use of an additive such as rubber have been employed to improve the impact resistance and the flexibility of polylactic acid. However, most of the products used for these purposes are derived from petroleum, thus compromising the feature of polylactic acid that it is derived from biomass.
In this respect, examples of a case where the impact resistance and the flexibility of polylactic acid have been improved by using a biomass-derived material include the production of a composite material of polylactic acid and natural rubber.
For example, Patent Documents 1 and 2 disclose resin compositions containing polylactic acid, natural rubber such as epoxidized natural rubber, and a carbodiimide compound. For the resin compositions of Patent Documents 1 and 2, it is described that the impact resistance has been improved by cross-linking the carbodiimide compound and the natural rubber. However, there is a concern that carbodiimide compounds may generate isocyanate gas during melt kneading or molding, resulting in a deterioration of the working environment. Further, the carbodiimide compounds used for these purposes are reagents that are generally not easily available, for example, in that they need to be set to have a predetermined molecular weight, and, therefore, it can be hardly said that they are suitable for industrial production.
In addition, Non-Patent Document 1 discloses that dynamically cross-linking of polylactic acid and natural rubber in a kneading device has improved the impact resistance of the resulting polylactic acid resin composition. However, for the polylactic acid resin composition, the natural rubber is added in excess in order to improve the impact resistance. Accordingly, it is inevitable that the mechanical properties of the polylactic acid itself are compromised. Further, there is also a concern that the natural rubber added may reduce the aesthetic properties of the resin molded article due to unnecessary coloration, or may cause latex allergy to the user.